The broad goal of this research project is to elucidate the mechanisms by which proteins and lipids in the endosome are packaged into transport carriers that bud from the endosome and ferry cargo to the Golgi apparatus and other organelles. These trafficking pathways, collectively termed 'endosomal retrograde transport,' are vital for cell physiology and homeostasis, metazoan development and they counter poorly understood processes that contribute to lysosomal storage diseases, cancer, Alzheimer's disease, and other diseases. In previous funding cycles we discovered that retrograde pathways are used to modulate the composition of the plasma membrane in response to environmental and nutritional cues, and we established roles for a soluble protein sorting complex called 'retromer' and three effectors of Phosphatidylinositol 3-kinase (PI3K) called sorting nexin (SNX) proteins. These components assemble on the endosome membrane where they capture and package cargo into budding transport carriers but the underlying mechanisms are unknown. With the principle cellular function of retromer now well established, the major challenge for the field is to elucidate the mechanisms by which it functions. Biochemical approaches will be used to dissect the mechanism of retromer assembly with cargo on the endosome membrane. These experiments will complemented with live cell imaging of retrograde transport that will place the assembly reactions in a physiological context. In recognition that the retromer sorting nexins are members of a much larger SNX protein family for which the functions of all but a few have been described, unbiased, proteome scale experiments will be used to elucidate the functions of each sorting nexin and to assess their specific contributions to cell physiology.